In order to create an efficient fuel cell based on a high-temperature proton conductor, it is necessary to develop a long-lived proton electrolyte. In the general case, the long-term chemical stability of the phase to CO2 is provided by thermodynamics (impossibility of reaction) or interaction kinetics (slowing down of the reaction). The paper compares the chemical stability with respect to CO2 (both thermodynamic and related to kinetic difficulties) for promising high-temperature proton conductors based on double perovskite Ba4-xCa2+xNb2O11 (x = 0.4; 0 ; -0.4) and double fluorite La6-xWO12-1.5 (x = 0.4, 0.6, 0.8, 1). The temperature of resistance to CO2 (above which the phase is stable to CO2, below which the phase interacts with CO2) is an important technical characteristic of the thermodynamic stability of the phase to CO2. The upper limit of the operating temperatures of the solid oxide fuel cell is 1,000ºC. When the temperature of stability is the lower, then the phase is more stable to CO2. We use solid-phase synthesis, X-ray diffraction, thermogravimetry with mass spectrometry and conductivity measurement by the impedance method. It is established that materials based on La6WO12 are relatively thermodynamically stable in ordinary air with CO2 (10-3 bar) in the operating range of 650-1,000 ºC. The phases based on Ba4Ca2Nb2O11 are resistant to CO2 in the air in the range of 850-1,000 ºC. In order to use the material in conditions of its thermodynamic stability, the stability temperature is required to be below the operating temperature (400-700 ºC). Thus, phases based on Ba4Ca2Nb2O11 are thermodynamically unstable to CO2 at 700ºC, and phases based on La6WO12 are stable at 700ºC. In the absence of thermodynamic stability of the phase, stability of this phase to CO2, associated with the kinetic difficulties, may be revealed in some cases, which is sufficiently long-term for practical use. For example, it is possible to form a continuous diffusion-blocking surface layer of products of interaction with CO2 (Ba, Ca, La carbonates) at the grain boundary of the main phase. The increase in the grain-boundary resistance observed for the studied samples may indicate the formation of a surface layer of products of interaction with CO2. For ceramic samples La6-xWO12-1.5x (x = 0.4, 0.6, 0.8, 1), the grain-boundary and electrode resistance after aging for 30 days at 200ºC in moist atmospheric air (CO2 10-3 bar) is shown to increase approximately 3 times at 800ºC and 10 times at 400 ºC.